MFC Windows 程序设计->捕获鼠标

井九

发布于 2024-10-12 08:39:21

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// In CMainWindow's message map ON_WM_LBUTTONDOWN () ON_WM_LBUTTONUP () void CMainWindow::OnLButtonDown (UINT nFlags, CPoint point) { SetCapture (); } void CMainWindow::OnLButtonUp (UINT nFlags, CPoint point) { ::ReleaseCapture (); }

In between, CMainWindow receives WM_MOUSEMOVE messages that report the cursor position even if the cursor leaves it. Client-area mouse messages continue to report cursor positions in client coordinates, but coordinates can now go negative and can also exceed the dimensions of the window's client area.

A related function, CWnd::GetCapture, returns a CWnd pointer to the window that owns the capture. In the Win32 environment, GetCapture returns NULL if the mouse is not captured or if it's captured by a window belonging to another thread. The most common use of GetCapture is for determining whether your own window has captured the mouse. The statement

if (GetCapture () == this)

is true if and only if the window identified by this currently has the mouse captured.

How does capturing the mouse solve the problem with the rubber-banded line? By capturing the mouse in response to a WM_LBUTTONDOWN message and releasing it when a WM_LBUTTONUP message arrives, you're guaranteed to get the WM_LBUTTONUP message when the mouse button is released. The sample program in the next section illustrates the practical effect of this technique.

void CMainWindow::OnLButtonDown (UINT nFlags, CPoint point)
{
    //
    // Record the anchor point and set the tracking flag.
    //
    m_ptFrom = point;
    m_ptTo = point;
    m_bTracking = TRUE;
    //
    // If capture is enabled, capture the mouse.
    //
    if (m_bCaptureEnabled)
        SetCapture ();
}
void CMainWindow::OnMouseMove (UINT nFlags, CPoint point)
{
    //
    // If the mouse is moved while we're "tracking" (that is, while a
    // line is being rubber-banded), erase the old rubber-band line and
    // draw a new one.
    //
    if (m_bTracking) {
        CClientDC dc (this);
        InvertLine (&dc, m_ptFrom, m_ptTo);
        InvertLine (&dc, m_ptFrom, point);
        m_ptTo = point;
    }
}
void CMainWindow::OnLButtonUp (UINT nFlags, CPoint point)
{
    //
    // If the left mouse button is released while we're tracking, release
    // the mouse if it's currently captured, erase the last rubber-band
    // line, and draw a thick red line in its place.
    //
    if (m_bTracking) {
        m_bTracking = FALSE;
        if (GetCapture () == this)
            ::ReleaseCapture ();
        CClientDC dc (this);
        InvertLine (&dc, m_ptFrom, m_ptTo);
        CPen pen (PS_SOLID, 16, RGB (255, 0, 0));
        dc.SelectObject (&pen);
        dc.MoveTo (m_ptFrom);
        dc.LineTo (point);
    }
}

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原始发表：2024-10-11，如有侵权请联系 cloudcommunity@tencent.com 删除

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